A turbofan gas turbine engine operates according to well known principles wherein an incoming air stream flows through the engine along an annularly configured, axially extending flow path. A portion of the incoming air stream is compressed in a compressor section of the engine and then mixed with fuel and burned in a combustor section to produce a high energy, high temperature exhaust gas stream. The gas stream exits the combustor and subsequently passes through a turbine section that extracts energy from the exhaust gas stream to power the compressor and produce bypass thrust by rotating a fan that acts generally on the remaining portion of the incoming air stream.
Uncontrolled leakage of gases within the engine contributes to reduced engine efficiency. Seals are used to control this energy loss by interposing them in a leakage path to reduce the volume or mass of the gas--atmospheric air, exhaust, or otherwise--passing from one part of the engine to the other. In the past engine seals have principally taken the form of labyrinth seals. The use of brush seals as a substitute for labyrinth seals is presently being investigated.
In general, brush seals are used to seal a leakage path between a pair of relatively movable members. A typical use for such seals in a gas turbine engine is where the seal will be placed in a leakage path between a stationary engine member and a rotating engine member, such as a shaft, to control the loss of air through the leakage path between the members. Brush seals are not intended to function so as to completely seal one engine section from another, but rather rely upon the torturous flow path created between the bristles to reduce the airflow from one part of the engine to another and to control the pressure drop between the engine parts.
Brush seals used in gas turbine engines comprise, in general, single or multiple seal stages with each stage including an annularly configured upstream, or high pressure, plate and an annularly configured downstream, or low pressure, plate. Together the plates sandwich therebetween an annular array of bristles known collectively as a bristle pack. Usually the bristles are disposed at about a forty-five degree angle to a radius drawn from the engine center line. The bristles are usually affixed at their radially outer end with their radially inner ends extending across into the leakage path between the engine members. The radially inner free ends of the bristles sealingly engage a sealing surface on the radially inner engine member, which is typically the rotating engine member.
Because the bristles are somewhat flexible, they are able to bend during an engine transient and still retain their sealing ability after the transient has passed. Examples of such transients include differential thermal growth between the engine parts, rotor/stator relative movement, and vibration of some sort. Thus, a rotating engine shaft, for example, may enter a vibration mode where the shaft is vibrating about its longitudinal axis, that is, when the shaft is rotating eccentrically. The leakage path that is sealed by the brush seal is defined by the annular gap between the downstream plate and the rotating member. This gap is made as small as possible, though not as small as desirable for sealing purposes, since the gap exists to accommodate the aforementioned engine transients.
The sealing efficiency of a brush seal over time is affected by the wear on the bristle ends contacting the sealing surface on the opposing engine part, as well as the overall contact of the bristle ends with the sealing surface. Worn bristles ends will dictate replacement of the seal or particular seal stage earlier than otherwise would be necessary, thereby increasing engine operating costs. Because the bristles are not directed along true radii to the engine center line, but rather are angled at about forty-five degrees relative thereto, excessive, irregular wear of the bristles may result from an adversary gas flow field, that is, a gas flow field that includes substantial velocity vectors disposed at angles other than perpendicularly to the seal. This adversary flow field can reduce the compactness of the bristle pack, which permits the individual bristles of the bristle pack to move randomly with a higher degree of freedom than the bristles of a seal not encountering such an adversary flow. When the bristles are tightly packed, they wear better over time and seal more efficiently. This enlarged freedom of bristle movement from the adversary flow field results in the bristles being displaced and rubbing, which in turn creates bristle wear.
Stated otherwise, any gas that encounters the seal that has a swirl, recirculation, or turbulence associated therewith will move the bristle ends and will contribute to unwanted bristle wear, often called tufting when originating from these causes. In common parlance, swirl is a rotational movement of the fluid molecules; recirculation is a radial movement of the fluid molecules; and turbulence is random, volatile movements of the fluid molecules. Thus, a radially outwardly directed recirculation, for example, can lift the bristles, which are usually attached at their radially outer but not their radially inner ends, thereby fluffing them and reducing their density. Additionally, an upstream jet flow, which forms part of the gas adversary flow, will vibrate the loosely packed or fluffed bristles and may open a small gap between the bristle ends and the sealing surface such that air can freely pass by an upstream seal stage and encounter a downstream seal stage with great velocity. This, in turn, will cause irregular wear on the downstream seal stage and may open a leakage gap at the adjacent downstream stage also, again allowing free passage of air or gas and greatly reducing engine efficiency. The lifetime and sealing efficiency of a brush seal are dependent in part on the chamfering of the bristle pack on the upstream side of the pack and wear of the bristles. This chamfering and wear facilitates movement of air through the bristle pack as well as blow-by conditions where a gap is opened between the bristle ends and the sealing surface on the rotating engine member.
Eccentric rotation of a rotor shaft can create unwanted bristle wear. Eccentric shaft rotation has been found to induce a one per revolution unsteady flow with respect to the stationary bristles. The unsteady flow causes the bristles to move as cantilever beams with the free ends of the bristles deflecting radially, tangentially, and axially. Radial deflection of the free ends increases the rubbing force between the free ends of the bristles and the sealing surface, thereby causing the free ends to wear. Because the amplitude of the induced vibration decreases axially from the inlet or upstream side of the seal to the outlet or downstream side of the seal, the upstream bristle free ends experience wear to a greater extent than the downstream free ends. Thus, the bristle pack is chamfered by the one per revolution induced excitation of the bristles.
To control bristle wear, it has been proposed to use dampers on the inlet face of the brush seal. Inlet dampers operate well to control bristle wear, but they are difficult to manufacture. To be effective, the dampers must engage the bristles with enough force to damp their vibrations, but must not engage them too tightly or the bristles can overheat during engine operation and even melt as a result of the overheating. In addition, any variations in the manufacturing process can lead to uneven damping of the bristle pack, thereby causing differential wear of the bristles forming the bristle pack.
It would be desirable to increase the lifetime and sealing efficiency of brush seals by reducing chamfering and wear of the bristle pack, by reducing seal damper manufacturing variations, and by manufacturing a damper that produces more uniform damping characteristics.